Reitveld: http://gwt-code-reviews.appspot.com/144805
Prevent a NullPointerException when JsDuplicateFunctionRemover encounters
a JsNameRef that does not have a JsName.

Patch by: cromwellian
Review by: spoon


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7593 8db76d5a-ed1c-0410-87a9-c151d255dfc7